The present invention relates generally to hydraulic couplings for use in motor vehicle driveline applications for limiting slip and transferring torque between rotary members. More specifically, a drive axle assembly for an all-wheel drive vehicle is disclosed having a pair of hydraulic couplings each having a fluid pump, a multi-plate clutch assembly, and a fluid distribution system operable to control actuation of the clutch assembly.
In all-wheel drive vehicles, it is common to have a secondary drive axle that automatically receives drive torque from the drivetrain in response to lost traction at the primary drive axle. In such secondary drive axles it is known to provide a pair of clutch assemblies connecting each axleshaft to a prop shaft that is driven by the drivetrain. For example, U.S. Pat. No. 4,650,028 discloses a secondary drive axle equipped with a pair of viscous couplings while U.S. Pat. Nos. 5,964,126, 6,095,939 and 6,155,947 each disclose a secondary drive axle with a pair of pump-actuated multi-plate clutch assemblies. In addition to these passively-controlled drive axles, U.S. Pat. No. 5,699,888 teaches of a secondary drive axle having a pair of multi-plate clutches actuated by electromagnetic actuators that are controlled by an electronic control system.
An object of the present invention is to provide a drive axle assembly equipped with a pair of hydraulic couplings which are operably arranged for coupling a vehicle drivetrain to a pair of axleshafts.
In carrying out the above object, the drive axle assembly of the present invention includes a drive case that is rotatably supported within a housing and driven by the drivetrain, first and second output shafts rotatably supported by the drive case, and first and second hydraulic couplings operably installed between the drive case and the first and second output shafts. Each hydraulic coupling includes a multi-plate clutch assembly and a clutch actuator. The clutch actuator includes a fluid pump and a piston assembly. Each fluid pump is operable for pumping fluid in response to a speed differential between the drive case and the corresponding output shaft. The piston assembly includes a piston retained for sliding movement in a piston chamber and a multi-function control valve. The fluid pump supplies fluid to the piston chamber such that a clutch engagement force exerted by the piston on the multi-plate clutch assembly is proportional to the fluid pressure in the piston chamber. The control valve is mounted to the piston and provides a pressure relief function for setting a maximum fluid pressure within the piston chamber. The control valve also provides a thermal unload function for releasing the fluid pressure within the piston chamber when the fluid temperature exceeds a predetermined temperature value.
In accordance with an optional construction, the multi-function control valve of the present invention can also provide a flow control function for regulating the fluid pressure in the piston chamber. The flow control function can further include a thermal compensation feature for accommodating viscosity variations in the fluid.